This invention relates, in general, to semiconductor devices, and more particularly, to methods for making semiconductor devices having electroplated leads.
Semiconductor packaged devices such as quad flat pack (QFP) devices typically comprise a semiconductor die bonded to a metal leadframe, which includes a die bond pad and a plurality of leads. Thin wires, typically referred to as wire bonds, connect bond pads on the semiconductor die to the appropriate leads. An encapsulent covers the semiconductor die, the wire bonds, and a portion of the leads. Exposed portions of the leads are often coated with a low temperature solderable metal, such as tin/lead (Sn/Pb), to enhance wetting or solderability and to form the metallurgical connection when the packaged device is connected to a printed circuit board during final assembly.
Typically, manufacturers coat the exposed leads using either a solder-dip process or an electroplating process. In a solder-dip process, the exposed leads are immersed in molten solder thereby covering the exposed leads. One disadvantage of the solder-dip process is that excess solder accumulates on the exposed leads, which causes solder bridging and ultimately electrical shorts. Because of the solder bridging, solder dipping is not suitable for devices having a small pitch between adjacent leads.
In an electroplating process, the leads are covered with a solderable metal by exposing them to an electrolytic solution, an anode comprised of the solderable metal, and a current source. Electroplating is preferred over solder-dipping in packages that have a small pitch between adjacent leads because electroplating provides a more uniform coverage. However, several problems exist with electroplated leads formed using conventional electroplating techniques.
For example, the electroplated solderable metal often exhibits poor adhesion to the underlying leads. This poor adhesion in turn causes the electroplated solderable metal to delaminate or peel from the underlying leads, particularly during subsequent processing (e.g., trim and form, electrical testing, tape and reel, etc.). The delaminated solderable metal detrimentally impacts equipment up-time (e.g., delaminated metal clogs or obstructs equipment), equipment life, electrical yields (e.g., electrical shorts resulting from delaminated metal bridging adjacent leads), and reliability. Equipment down-time decreases throughput thereby increasing manufacturing costs. In addition, the delamination problem requires manufacturers to use visual inspections at various stages after electroplating to control problem parts, which greatly adds to manufacturing costs. Furthermore, localized thick areas or nodules of solderable metal form during electroplating, which can result in bridging between adjacent leads and ultimately electrical shorting problems.
As is readily apparent, it would be advantageous to have methods that overcome at least the above problems associated with leads having electroplated solderable metal. It would be of further advantage to do so in a cost effective manner.